Major depressive disorder (MDD) has been associated with abnormally reduced function of central serotonergic systems by various types of evidence. One instructive paradigm for investigating the relationship between serotonergic function and depression has involved the mood response to tryptophan depletion (TD), achieved by oral loading with all essential amino acids excepting the 5-HT precursor, tryptophan. We obtained preliminary evidence that the mood lowering effect of TD depends upon the genotype for a functional polymorphism in the promoter region of the 5-HT transporter (5-HTT), designated 5-HTTLPR, as well as upon family history. In healthy females the s-allele and a positive family history for mood disorders appeared to be additive risk factors for the development of depressive symptoms during TD. The current study employed quantitative PET imaging of cerebral blood flow (CBF) and glucose metabolism to investigate the effect of variant 5-HTTLPR genotypes on the neurophysiological response to TD. We also examined the relationship between 5-HTTLPR genotypes and the TD effect on PFC metabolic activity and whether reduction in PFC metabolism in response to TD would occur to a greater extent in subjects with the s/s allele and in subjects with a single s allele plus a family history of depression. We also examined whether this reduction in PFC metabolic activity is unique to subjects who develop depressive symptoms during TD. In addition, based upon evidence that 5-HT inhibits neuronal activity in the amygdala, and modulates transmission of emotionally-salient sensory information from the sensory cortices to the amygdala, we tested the hypothesis that in MDD, reduced serotonin function associated with TD may disinhibit the amygdaloid response to sensory stimulation. This hypothesis was explored by assessing the physiological responses of the amygdala to sensory stimuli that normally activate the amygdala, namely pictures of human faces that show fearful or sad emotional expressions. We were particularly interested in determining whether the amygdala CBF response to emotional stimuli during TD is most prominently increased in subjects carrying the s-allele of the 5-HTTLPR, and whether it is unique to subjects who develop depressive symptoms during TD. A new project was added this year to examine the relationships between genotype, history of MDD, and serotonin depletion on the processing of emotionally valenced words. Recovered subjects with a history of MDD and healthy controls underwent functional MRI scanning while performing an "affective shift task" (in which subjects alternate attention between detecting sad versus happy words) in both the tryptophan depleted and the control condition. Thirty-six unmedicated-remitted subjects with MDD and 36 healthy controls were studied in previous years in a double-blind, placebo-controlled, randomized (according to 5-HTTLPR genotype) crossover study. The data analysis has continued and a manuscript has been prepared for submission to describe the effects of genotype on the brain regions that change their activity during serotonin depletion and during depressive relapse.